More fixes for 'volatile' and setjmp/longjmp.

More fixes for 'volatile' and setjmp/longjmp.

* eval.c (Fdefvar, Fcondition_case): Remove unnecessary 'volatile's.
* image.c (struct png_load_context) [HAVE_PNG]: New type.
(png_load_body) [HAVE_PNG]:
(jpeg_load_body) [HAVE_JPEG]:
New function, with most of the old parent function's body.
(png_load) [HAVE_PNG]:
(jpeg_load) [HAVE_JPEG]:
Invoke the new function, to avoid longjmp munging our locals.
(struct my_jpeg_error_mgr) [HAVE_JPEG]: New members cinfo, failure_code.
(my_error_exit) [HAVE_JPEG]: Don't trust 'setjmp' to return 2 when
longjmp is passed 2, as the C standard doesn't guarantee this.
Instead, store the failure code into mgr->failure_code.